Effects of environment on dry sliding wear of powder metallurgical Ti-47Al-2Cr-2Nb-0.2W

Intermetallics 53 (2014) 10e19

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Effects of environment on dry sliding wear of powder metallurgical Ti-47Al-2Cr-2Nb-0.2W J. Qiu a, b, Y. Liu a, *, F. Meng b, I. Baker b, **, P.R. Munroe c a

State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083, China Thayer School of Engineering, Dartmouth College, 14 Engineering Drive, Hanover, NH 03755-8000, USA c Materials Science and Engineering, UNSW Australia, Sydney, NSW 2052, Australia b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 December 2013 Received in revised form 15 February 2014 Accepted 25 February 2014 Available online

The effects of environment on the dry sliding wear behavior of Ti-47Al-2Cr-2Nb-0.2W (at.%) prepared by hot isostatic pressing (HIP) of powders have been investigated. Room temperature pin-on-disk tribotests were performed against an yttria-stabilized zirconia counterface in four different environments: air, oxygen, 4% hydrogen in nitrogen, and argon. It was found that this alloy is sensitive to the presence of oxygen, i.e. the pins had much lower wear rates in the oxygen-free environments. The presence of water vapor and molecular hydrogen had little effect on the wear rate. The effects of the different sliding speeds were also studied in air and argon and it was found that slower sliding rates led to greater wear of the pins, particularly in air. The tips of the worn pins were examined using both scanning electron microscopy and transmission electron microscopy, the latter using specimens produced by focused ion beam milling. At the higher sliding speed, the abrasive particles largely consisted of the counterface material and the zirconia particles were found to be embedded in a tribolayer on the worn tips of the pins. At the lower sliding speed, the abrasive particles mainly arose from the pins. The results indicate that both twobody and three-body abrasive wear, as well as plastic deformation and delamination, were the main wear mechanisms. Phase transformations in both the pin and the counterface at the contact area probably also played a role in the wear processes. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: A. Titanium aluminides, based on TiAl B. Tribological properties F. Electron microscopy, transmission

1. Introduction Gamma TiAl-based alloys are considered to be promising hightemperature lightweight structural materials due to their high specific yield strength, outstanding high specific stiffness, and good creep properties at high temperatures [1e3]. Some promising applications include components for automobile engines, such as exhaust valves, tappets and valve spring retainers [4]. Turbochargers manufactured from Ti-46Al-6.5Nb have equipped more than 20,000 cars in Japan in 2003 [2]. Practical applications have been extended to the low pressure turbine blades for jet engines in order to achieve improved efficiency [5]. However, high processing costs and segregation in large ingots limit the applications of as-cast materials. Wu [2] suggested that powder metallurgy (P/M) processing is potentially important,

* Corresponding author. Tel.: þ86 731 88836939. ** Corresponding author. Tel.: þ1 603 646 2184. E-mail addresses: [email protected], [email protected] (Y. Liu), Ian. [email protected] (I. Baker). http://dx.doi.org/10.1016/j.intermet.2014.02.021 0966-9795/Ó 2014 Elsevier Ltd. All rights reserved.

especially for large products. In addition, pre-alloyed powder is particularly attractive since it provides good compositional uniformity, strength and ductility. P/M TiAl alloys also do not need subsequent thermomechanical process to obtain a fine and homogenous microstructure [6]. The wear resistance of titanium aluminide is another challenging factor which has to be considered in some applications [4]. There are extensive efforts to improve the mechanical properties and oxidation resistance of g-TiAl-based alloys by alloying and optimization of the microstructures [7e9]. Several workers [4,10,11] have studied the effects of different counterfaces, microstructures, coatings, temperature and loading forces on the tribological behavior of TieAleNbeCr-based alloys. The different wear debris produced by different counterfaces also influence the wear rate of TiAl [4]. Gialanella [10] found that oxidation improves the surface durability of gamma TiAl alloys by conventional, comparatively cheap heat treatment before the wear test. Cheng [11] pointed out that the wear mechanism of the Ti-46Al-2Cr-2Nb is strongly dependent on the experimental temperature. However, little work has been conducted on the wear behavior of TiAl alloys in different environments. This paper focuses on the effects of the

J. Qiu et al. / Intermetallics 53 (2014) 10e19

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reactive and does not contain any of the elements in the wear pin. The latter feature enables the origin of the elements in the tribolayer produced after wear testing to be determined unambiguously. The disc material, which was obtained from SaintGobain Advanced Ceramics, is a sinter-HIPed grade of zirconia that is partially stabilized to tetragonal zirconia with 2.8 mol% (4.8 wt.%) yttria. All tests were conducted on a new disk surface at

Sliding distance(m) 1.0

0

200

400

600

800

1000

(a)

Ar H2 O2 Air

sliding speed: 1m/s Friction coefficient

0.8

0.6

Fig. 1. BSE images of the as-HIPped Ti-47Al-2Cr2Nb-0.2W (at.%). 0.4

environment and sliding velocity on the dry sliding wear of P/M Ti47Al-2Cr-2Nb-0.2W (at.%) against an yttria-stabilized zirconia counterface.

0

200

400

600

800

1000

800

1000

Time(second)

2. Experimental

Sliding distance(m) 0 1.2

400

600

(b)

Ar Air sliding speed: 0.1m/s

1.0

Friction coefficient

Powder with a nominal composition of Ti-47Al-2Cr-2Nb-0.2W (at.%) was produced by plasma rotating electrode process (PREP), canned in steel and hot isostatically pressed (HIP) at 1523 K for 5 h with a pressure of 170 MPa. Cylindrical pins of 9.5 mm diameter were prepared by electro-discharge machining from the HIPped ingot. Hemispherical tips were produced on the pins by turning on a lathe, after which the tips were polished to a mirror finish using 2000 mesh SiC paper. The microstructure of the alloy is mostly g phase with small amount of a2/g lamellae phase, see Fig. 1. Pin-on-disc wear tests were performed against a 10 cm dia. yttria-stabilized zirconia counterface polished to a surface finish of w0.1 Ra(w0.0254 mm), using a home-made device [12]. Yttriastabilized zirconia was used as the counterface since it is non-

200

0.8

0.6

0.4

0.2 0

2000

4000

6000

8000

10000

Time (Second) 0.025

1.0

0.8

0.1m/s

0.015

0.010 1m/s

Friction coefficient

Weight loss/g

0.020

(c)

0.6

0.4 1m/s

1m/s 1m/s

0.1m/s

1m/s

0.2

0.005

0.1m/s

1m/s

0.1m/s

1m/s 1m/s

0.0

0.000 Argon

Hydrogen Oxygen

Air

Argon 0.1m/s

Air 0.1m/s

Fig. 2. Mean mass loss for Ti-47Al-2Cr-2Nb-0.2W (at.%) pins in four different environments i.e. air, argon, 4 vol.% hydrogen in nitrogen and oxygen after sliding for 1 km using different sliding speeds.

Argon

Hydrogen

Oxygen

Air

Argon 0.1m/s

Air 0.1m/s

Fig. 3. Friction coefficient vs. time/sliding distance in different atmospheres and sliding speeds: (a) argon, hydrogen, oxygen and air, 1 m/s, (b) argon and air, 0.1 m/s, and (c) average friction coefficient (steady state) with standard deviations (from three tests) of Ti-47Al-2Cr-2Nb-0.2W (at.%) in different environments.

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J. Qiu et al. / Intermetallics 53 (2014) 10e19

Amorphous TiAl Air,0.1m/s

Intensity(Counts)

(111)

1000

(220) (200)

TiAl

(311)

(331)

(222) (400)

Cubic ZrO

(422)

(420)

(333)

(440) (531)

O , 1m/s H , 1m/s Ar, 1m/s Air, 1m/s

20

40

60

80

100

120

140

160

2θ° Fig. 4. X-ray diffraction patterns from debris collected during wear tests of Ti-47Al2Cr-2Nb-0.2W (at.%) pins tested in different atmospheres and sliding speeds. The positions of possible cubic ZrO2 diffraction peaks are indexed.

room temperature for a total sliding distance of 1 km with a normal load of 23 N in four different environments, i.e. oxygen, air, argon and 4% (vol.%) hydrogen in nitrogen mixture. Three tests were performed in each environment. The humidity in oxygen, argon and the hydrogen/nitrogen mixture was <5%, while the humidity of the air was w45%. The wear tests were also separately performed at two different sliding speeds, 1 m s1 in all four different atmospheres and 0.1 m s1 in air and argon. Debris was collected during the wear tests using adhesive tape wrapped around the outside of the zirconia disk.

Wear mass loss was determined by measuring the mass difference of the pins before and after a wear test using an electronic balance of 0.1 mg precision. The phases present in the debris and worn surface on the pin were analyzed using a Rigaku D/Max 2000 X-ray diffractometer using Cu Ka radiation operated at 40 kV and 300 mA. Measurements were performed by stepped scanning from 20 to 140 with a step size of 0.02 . A count time of 1 s per step was used, giving a total scan time of w 1.5 h. The morphology of both the debris and the worn surfaces of the pins were examined using secondary electron (SE) imaging in an FEI XL-30Ô field emission gun scanning electron microscope (SEM), equipped with an EDAX Li-drifted energy dispersive X-ray spectrometer (EDS). Cross-sectional transmission electron microscope (TEM) specimens, used to examine the subsurface of the wear pins, were prepared using a FEI Nova 200 Nanolab focused ion beam microscope (FIB) using the lift-out method [13,14]. The specimens were examined using a Philips CM200 TEM operating at 200 kV. Elemental X-ray maps were collected in scanning transmission electron microscope (STEM) mode using EDS. The worn tracks of the zirconia disk were examined using a Zygo Newview 7300 3D optical surface profiler to record the profile readings. The profilometer data gave the depth readings for a crosssection of the wear track. 3. Results The mean weight loss for the pin-on-disk tests in four different environments and different sliding speeds after 1 km sliding tests with a 23 N normal load are shown in Fig. 2. Error bars signify standard deviations for the three samples per test condition. The wear loss of the Ti-47Al-2Cr-2Nb-0.2W alloy in the oxygen-free environments (argon and hydrogen) is much less than in the oxygen-containing environments (air and oxygen). Also the

Fig. 5. SE images of debris wear pin and corresponding EDS spectra marked by square and arrow, from a TiAl tested in (a) (b) air, 1 m/s, (c) (d) argon, 0.1 m/s.

J. Qiu et al. / Intermetallics 53 (2014) 10e19

tribological behavior of Ti-47Al-2Cr-2Nb-0.2W alloy is not sensitive to dry hydrogen gas. Liu [15] pointed out that many intermetallic alloys containing reactive elements like AI, Si, Ti and V show environmental embrittlement at ambient temperatures. In the case of TiAl, both titanium and aluminum are considered to react with moisture in air, rather than hydrogen available from hydrogen gas, resulting in the generation of atomic hydrogen producing embrittlement. Interestingly, the wear loss at the slow sliding speed of 0.1 m/s is larger than that at 1 m/s in the same atmosphere. The friction coefficients, which were recorded every 30 s during the wear tests, are shown as a function of time along with quadratic fits to the data in Fig. 3(a) and (b). From the curves of the polynomial fit, it can be found that the friction coefficient decreased from a higher initial value to a lower steady state value in most conditions except in the air at 1 m/s Fig. 3(c) shows the average friction coefficients (steady state) along with the standard deviations from three repeat tests for each environment and sliding speed. The friction coefficients in the oxygen-containing environments (air, oxygen) are slightly lower than those in oxygen-free environments (argon, hydrogen/nitrogen) at the sliding speed of

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1 m/s, with those in oxygen being the lowest. The latter suggests that the water vapor in the air affects the friction coefficient. When the sliding speed is 0.1 m/s, the friction coefficients in air are larger than in those in argon, with those in argon being significantly lower than those measured at the higher sliding rate. Fig. 4 shows X-ray data from the debris of the wear tests in different atmospheres and at different sliding speeds. It was not possible to perform a similar analysis for the tests conducted under argon at 0.1 m/s due to the much smaller quantity of debris present. It can be observed in Fig. 4 that the debris from the wear tests at 1 m/s consists mainly of cubic zirconia with a little TiAl. When the sliding speed is 0.1 m/s, the wear tests in the air mainly produced amorphous TiAl. Debris with a range of sizes and irregular shapes are shown in Fig. 5. The EDS results in Fig. 5(c) and (d) also confirm that there is little zirconia in the debris at the sliding speed of 0.1 m/ s. The morphologies of the worn surfaces of the pins are shown in Fig. 6. The wear pits, long parallel wear grooves marked by the ellipses, are evidence of abrasive wear. Some of the wear particles on the worn surface, as shown in Fig. 6(b), are consistent with those

Fig. 6. SE images of the worn surface of TiAl alloys after pin-on-disk wear tests in different atmospheres and at different sliding speeds: (a), (b) in air, 1 m/s; (c) in oxygen, 1 m/s; (d) in argon, 1 m/s; (e) in argon, 0.1 m/s and (f) in air, 0.1 m/s. The arrows indicate the direction of sliding.

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J. Qiu et al. / Intermetallics 53 (2014) 10e19 20

(a)

10

10

0

0

Depth (μm)

depth (μm)

20

-10

Air Argon Hydrogen Oxygen

-20

-30 0

1

2

3

4

5

(b)

Air-slow Ar-slow

-10

-20

-30 0

1

width (mm)

2

3

4

5

Width (mm)

Fig. 7. Profilometer traces across wear tracks on zirconia disk.

in abrasive wear processes. Many debris particles adhered to the surfaces are also shown in Fig. 6(a), (b) and (d). It is worth noting that there were some zirconia particles attached on the worn surface of TiAl pins tested at 1 m/s in all four different atmospheres. However, it was found that there are no foreign materials on the worn surface at the sliding speed of 0.1 m/s, as evident from the back-scattered electron images shown in Fig. 6 (e) and (f). Typical wear tracks on the zirconia disk, examined using a 3D optical surface profiler, are shown in Figs. 7 and 8. When the sliding speed is 1 m/s, clear wear tracks were left on the zirconia disks after wear tests in all atmospheres except oxygen, see Fig. 7(a). However, as shown in Fig. 7(b), irrespective of atmosphere, there were no obvious wear tracks on the zirconia disk at the sliding speed of 0.1 m/s. This is consistent with the lack of oxide formation on the

pins in Fig. 6 (e) and (f), which are thus not hard and don’t produce third party wear. The 3D maps in Fig. 8 also confirm that the wear tests in the air at 1 m/s left much deeper wear tracks. It is widely accepted that the wear resistance under dry sliding wear conditions is closely related to the formation and the stability of tribolayers on the contact surfaces [16e20]. TEM specimens were extracted from the worn surface of the pins using FIB microscopy. Fig. 9 shows pits formed using the FIB, from which TEM specimens were taken. A platinum strip (labeled “A”) was deposited on the surface to protect it during ion milling. There are two layers below the protective platinum strip (labeled “B” and “C”) when the sliding speed is 1 m/s, as shown in Fig. 9(a) and (c). While the tribolayer in Fig. 9(b) at 0.1 m/s has only one layer, the TiAl base alloy. Fig. 10 shows bright field TEM images of the material close to the worn surface of the pins tested in air at 1 m/s and 0.1 m/s. A mixed layer was found only in Fig. 10(a), which is consistent with the B layer in Fig. 9(a). The selected area diffraction pattern in Fig. 10(c) confirms that cubic zirconia was embedded into the mixed layer. Fig. 11 and Fig. 12 show X-ray maps of the elemental distributions in the corresponding zones of Fig. 10(a) and (b). As shown in Fig. 11(a), when the sliding speed is 1 m/s, a thick layer that mainly consists of zirconia is attached to the worn surface. Some zirconia particles embedded below the worn surface are marked by ellipses in Fig. 10 (a) and Fig. 11 (a). Chromium and niobium were also incorporated, probably as oxides, into the tribolayer, as shown in Fig. 11(f) and (g). Taken together with Fig. 10(b), Fig. 12(a) confirms that there is nearly no zirconia in the tribolayer when the sliding speed is 0.1 m/ s. But some oxides of chromium, niobium and aluminum may be found on the worn surface. 4. Discussions

Fig. 8. 3D wear tracks on zirconia disk after wear test in air at (a) 1 m/s and (b) 0.1 m/s.

The friction and wear behavior is related to material’s mechanical properties at different temperatures and the wear debris or tribolayer on the worn surfaces generated during sliding. Due to the frictional heating, the local temperature at the sliding interface is higher than the ambient temperature and may be enhanced at the asperity contacts by transient “flashes” or “ hot spots”. During the wear tests at 1 m/s occasionally sparking was also observed (even with room lights on) between the pin and the disk. An approximate calculation was performed of the contact temperatures resulting from the frictional heating within the assumed Hertzian contact area at the sliding interface of TiAl alloys [21]. Here it should be noted that the temperature analysis in the appendix is a relatively conservative estimate of the contact

J. Qiu et al. / Intermetallics 53 (2014) 10e19

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Fig. 9. SE images from FIBbed cross-section of worn TiAl pins after wear test in (a) air, 1 m/s, (b) air, 0.1 m/s, and (c) argon, 1 m/s.

Fig. 10. Bright field STEM image of TiAl pin worn in the air (a) at 1 m/s, (b) at 0.1 m/s. (c) Selected-area diffraction pattern from the dark phase marked “A” in the mixed layer.

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Fig. 11. X-ray elemental maps of the STEM image of Fig. 10(a) for (a) zirconium, (b) platinum, (c) titanium, (d) oxygen, (e) aluminum, (f) chromium, and (g) niobium.

temperature at the interface between the hemispherical shaped TiAl and the flat zirconia disk, since a single Hertzian (elastic) contact was assumed. In practice, it is probable that at any instant the real area of contact within the Hertzian region consisted of a finite number of concentrated asperity-level contacts. The peak contact temperature rise could be considerably higher at localized asperity contacts, but these would be of shorter duration [13].

As shown in Table 2 of the Appendix, these contact temperatures from the frictional heating during the wear test at 1 m/s are all above 1000  C, and sufficient to induce oxidation. Localized temperatures may become significantly higher in concentrated asperity contacts. However, mechanical forces can easily fracture the oxide layer during sliding so that the oxide layer would eventually crack and flake off from the surface, leaving behind pits. The oxide layer,

J. Qiu et al. / Intermetallics 53 (2014) 10e19

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Fig. 12. X-ray elemental distribution maps of the STEM image of Fig. 10(b) for (a) zirconium, (b) platinum, (c) titanium, (d) oxygen, (e) aluminum, (f) chromium, and (g) niobium.

attributed to high contact temperature and oxygen, does not seem to be effective as a protective layer on the worn surface. The brittle and hard oxide particles break off from the pin surface and tumble in the space between the surfaces. The hard aluminum oxides remaining on the pin surface could act as abrasive asperities and abrade the zirconia counterface and produce zirconia particles in the wear debris. At the same time, both the removed oxides and the

zirconia particles become abrasive third bodies and can remove materials from both the pin and the zirconia disk. The zirconia embedded in the tribolayer is widely found on the TiAl pins after the wear tests using 1 m/s as a sliding speed in different atmospheres, except for pure oxygen. As shown in Table 2 in the Appendix, the contact area on the zirconia disk was subject to high temperature and high localized repetitive shear stress, likely

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J. Qiu et al. / Intermetallics 53 (2014) 10e19

resulted in phase transformation, accompanied by surface uplifting, microcracking, and grain pullout [17,22]. The cubic-zirconia peaks from debris collected during wear tests at 1 m/s in Fig. 4 confirm that the original tetragonal zirconia disk had a stress- and temperature-induced phase transformation from a tetragonal to a cubic phase during the wear tests. This phase transformation will result in a volume change, that can cause the cracking within the zirconia crystals, even in the yttria-stabilized zirconia [22e24]. Since zirconia is much harder than TiAl, zirconia particles will be prone to become embedded into the worn surface on the TiAl pins. After the initial period of the wear tests, the zirconia particles were embedded into the tribolayer and operated as a protective layer during the wear test. Then more zirconia will be worn off from the disk by itself at that time. A sufficient contact temperature is the essential condition for producing enough zirconia particles which can become embedded into the tribolayer. Therefore, the contact temperatures of the wear tests at 0.1 m/s are well away from those that induce phase transformation of zirconia on the wear disk. This may explain why there was nearly no zirconia in the debris and the tribolayers on the TiAl pins from the wear tests at 0.1 m/s. Without the protection of the zirconia tribolayer, the TiAl will be easier to be worn by the hard zirconia disk, since the zirconia disk is much harder than TiAl. There is nearly no zirconia present to act as a protective layer on the TiAl pins in the oxygen atmosphere at 1 m/s in Fig. 6(c). As shown in Fig. 7(a), the wear track on the zirconia disk is also very small after the wear test in oxygen, although some cubic zirconia peaks can be found in the X-ray diffraction patterns in Fig. 4. Compared with other three contact temperatures at 1 m/s, the lower contact temperature in the oxygen may not produce enough zirconia particles to be readily detected as described above. The pure oxygen and high contact temperature also enhanced the oxidation of the TiAl. Both factors mean that the wear loss in air is greatest at the sliding speed of 1 m/s. However, as described in the Results section, when the sliding speed is 0.1 m/s, there is almost no zirconia embedded as a protective layer on the worn surface of TiAl pins. On the other hand, the total sliding time of the wear test at 0.1 m/s is 10 times longer than at 1 m/s, the effect from environmental embrittlement will be enlarged. Based on this condition, without the protection of zirconia layer, it is not surprising that the wear loss in the air at 0.1 m/s is the largest. As discussed above, the environment has significant effects on the mechanical properties of the TiAl pins and chemical state on the worn surface. The structural features and chemical state of the worn surfaces play important roles on the friction coefficient. From Fig. 3(c) it is evident that friction coefficients are sensitive to the presence of oxygen. When the sliding speed is 1 m/s, the friction coefficients in the oxygen-containing environments are a little lower than in the oxygen-free environments. The slightly lower friction coefficient may be explained by the lubrication effect of the TiO2x formed on the worn surfaces in the oxygen-containing environments [11,25,26]. Apart from the effect of oxygen, when the atmosphere is argon, the friction coefficient of the wear test at 0.1 m/s is much lower than at 1 m/s. The higher contact temperature from the high sliding speed may be the main reason. Because of the zirconia phase transformation in the contact area of the disc at the high contact temperature, numerous hard zirconia particles were attached on the worn surface of the pin as shown in Fig. 6 (d), bringing more asperities into contact during the sliding wear. 5. Conclusions The dry sliding wear behavior of Ti-47Al-2Cr-2Nb-0.2W (at.%) was investigated using pin-on-disk tribotests at sliding speeds of 1 m/s and 0.1 m/s for a 1 km sliding distance with a normal load of

23N against a zirconia disk in four different environments, i.e. air, oxygen, 4% hydrogen in nitrogen and argon. Based on the experimental results, the following conclusions can be made: 1. Irrespective of environment, zirconia particles are embedded into a tribolayer on the wear pins by plastic deformation of the TiAl surface if the contact temperature is high enough and the sliding speed is high. 2. To some extent, the hard tribolayer with the zirconia particles protects the TiAl pin from being worn by the hard zirconia disk. 3. The wear rate of the TiAl alloys is sensitive to the oxygen content of the testing environment. The effect of hydrogen and water vapor on the wear rate is minor. 4. The dominant wear mechanisms of the TiAl are two-body and three-body abrasive wear, as well as plastic deformation and delamination under some conditions. Acknowledgments JQ and YL would like to thank Financial Supports of National Key Fundamental Research and Development Project of China (2011CB605505) and acknowledge the help of Dr.Charles Daghlian in microscopy. IB and FM were supported by the US Department of Energy (DOE), Office of Basic Energy Sciences grant DE-FG0207ER46392. The zirconia disc was graciously provided by Dr. OhHun Kwon of Saint-Gobain Research and Development Center, Northboro, MA. Appendix Here we calculate the contact area of the pin and the contact temperature at the end of the wear pin. All tests were conducted on a new disk surface at a constant sliding speed of 1 m s1 for a total sliding distance of 1 km with a normal load, w, of 23 N. The radius, r, of the TiAl pins was 4.75 mm.

Table 1 Material properties of TiAl alloys and zirconia used in the wear tests. The data of partially yttria-stabilized zirconia disks indicated by þ come from Norton-Saint Gobain Company [27]. The properties indicated by # were from Ref. [28], while the remainder were measured. (The Poisson’s ratio of TiAl is the theoretical value.) Material/property

ZrO2

Ti-47Al-2Cr -2Nb-0.2W (at.%)

Hv, Hardness (GPa) E, Modulus of Elasticity (GPa) r, Density (kg/m3) n, Poisson’s ratio K, Thermal Conductivity (W/m K) C, Specific heat (J/Kg K)

13.5þ 205þ 6070þ 0.25þ 2.0þ 630#

2.71 143.767 4056 0.33 14.96 e

The radius of the contact circle, assuming Hertzian contact [21], is given by

a ¼




3wr 4E’

1=3

where

1  n21 1  n22 1 ¼ þ ’ E1 E2 E For a moving flat zirconia disc (material 1) and stationary unworn wear pin (material 2), the contact temperature rise due to

J. Qiu et al. / Intermetallics 53 (2014) 10e19

frictional heating for Hertzian elastic contact (following the methodology of [29]) is given by

2:31ampV DTmax ¼ pffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi p K1 1:2344 þ Pe1 þ K2 1:2344 þ Pe2 The Peclet numbers are Pe1 ¼ V1$a$r1$C1/2K1; Pe2 ¼ V2$a$r2$C2/ 2K2 ¼ 0, and the friction coefficients, m, are shown in Fig. 3. The average contact pressure,p ¼ w=a2 . The tests were performed at room temperature (25  C). Neglecting any nominal surface temperature rise due to repeated contact, the surface temperatures at the center of the contact area of the TiAl pins in different environments are Tcontact ¼ DTmax þ 25  C [29]. The results are shown in Table 2. Note: The DTmax are short duration temperature rises. Table 2  Contact temperature ( C) of TiAl pins tested against an yttria-stabilized zirconia disk at room temperature in four different environments. Atmosphere/ sliding speed

Argon

Hydrogen/ Nitrogen

Oxygen

Air

1 m/s 0.1 m/s

1820 119

1746 N/A

1109 N/A

1361 314

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